JP4887889B2 - Autofocus device and camera - Google Patents

Description

  The present invention relates to an autofocus apparatus that performs so-called contrast-type autofocus (AF) control, and a camera having the autofocus apparatus, and is intended to shorten the focusing time.

  In the contrast method, a subject luminous flux is received and subjected to photoelectric conversion, and a focus evaluation value corresponding to the contrast is calculated from the photoelectric conversion output (image signal). In view of the fact that the closer the focusing lens is to the focusing position, the higher the contrast of the image, the lens position where the focus evaluation value is maximized (peak) is found, the focusing lens is driven, and the subject is focused. For detection of the peak position, a so-called hill-climbing control is used (see, for example, Patent Document 1).

JP 2004-280048 A

  Contrast-type autofocus control can obtain a highly accurate focus adjustment result, but has a drawback that it takes time to focus.

The present invention includes a photoelectric conversion unit that receives and photoelectrically converts a subject light beam, and a calculation unit that calculates a focus evaluation value representing a focus adjustment state based on the photoelectric conversion output, and a focusing lens is set to a predetermined value. It is applied to an autofocus device that calculates the focus evaluation value at each sampling position while driving at every sampling pitch, and brings the focusing lens to the focusing position based on the calculation results, and the photoelectric conversion means is provided on the focal plane. A first photoelectric conversion element having a first light-receiving surface, and a second photoelectric conversion element having a second light-receiving surface provided to be movable in the optical axis direction with respect to a surface equivalent to the focal plane. A control unit that controls the position of the second light receiving surface to a predetermined position having an optical path length difference from the first light receiving surface according to the driving direction of the focusing lens, and the focusing lens is moved in the driving direction by the sampling pitch. Drive Occasionally, wherein the second photoelectric conversion element detects the peak of the focus evaluation value before the first photoelectric conversion elements.
The camera according to the present invention includes the autofocus device.

  According to the present invention, in contrast-based autofocus control, the focus position can be estimated before the focus evaluation value of the focus lens reaches the peak position, so that the speed of autofocus can be increased.

An embodiment of the present invention will be described with reference to FIGS.
FIG. 1 is a schematic configuration diagram of an electronic camera according to the present embodiment, and shows only a focus-related portion. The lens barrel 100 includes a photographic optical system 10 including a focusing lens 11, and a subject light beam that has passed through the photographic optical system 10 is guided to the camera body 200. The camera body 200 is provided with a half mirror 21, and the subject luminous flux reflected by the mirror 21 is received by the first photoelectric conversion element 22.

  The first photoelectric conversion element 22 photoelectrically converts the received light beam and outputs an electrical image signal. Image data is generated by performing various processes on the image signal, and is recorded as an image file on an image recording medium (not shown). The image signal that is the output of the first photoelectric conversion element 22 is input to the CPU 24 via the first focus detection means 23.

  On the other hand, the subject luminous flux transmitted through the half mirror 21 is received by the second photoelectric conversion element 25. The second photoelectric conversion element 25 can be moved in the direction of the incident optical axis by the photoelectric conversion element driving means 26. As the photoelectric conversion element driving means 26, a piezoelectric element, a voice coil motor, a minute stepping motor, or the like can be used. The photoelectric conversion output of the second photoelectric conversion element 25 is input to the CPU 24 via the second focus detection means 27.

  The display means 28 is composed of, for example, a liquid crystal monitor provided on the back of the camera, or an electronic viewfinder, and its drive circuit. The display unit 28 and the photoelectric conversion element driving unit 26 are driven and controlled by the CPU 24. The CPU 24 performs AF control based on the image signals input from the first and second focus detection units 23 and 27 and drives the focusing lens 11 through the lens driving unit 12 for focusing.

Details of the AF control will be described below.
The camera of the present embodiment performs contrast AF control (contrast AF). In contrast AF, a high-frequency component in a predetermined band is extracted from an image signal obtained by imaging, and the focus evaluation value is calculated by integrating the absolute value. The focus evaluation value is an amount that changes according to the contrast of the image, and the value increases as the contrast increases. The closer the focusing lens 11 is to the in-focus position, the higher the contrast of the image signal. Therefore, by bringing the focusing lens 11 to a position where the focus evaluation value reaches a peak (maximum), the subject can be focused. . The operation for obtaining the lens position at which the focus evaluation value reaches a peak is called AF search.

A hill-climbing method is known as an AF search method, which will be described with reference to FIG.
FIG. 2 is a diagram showing the change of the focus evaluation value with respect to the focus lens position. Now, the focus lens 11 is at the P (1) position, and the focus evaluation value at that time is a (1). P (n) (n = 1, 2,...) Is a sampling point, and a focus evaluation value is calculated at this point. In this example, if the focusing lens 11 is located in a predetermined region centered on the peak position of the focus evaluation value, it is assumed that the focus is achieved.

  FIG. 2 shows a (n) and b (n), but only a (n) is used in the conventional contrast AF. That is, first, in order to determine the lens driving direction, the focusing lens 11 is moved in any direction by the sampling pitch to obtain the focus evaluation value a (n). For example, when driving to the P (2) position, the obtained focus evaluation value a (2) is compared with a (1), and a (1) <a (2). I understand that. Therefore, the focusing lens 11 is further moved to the infinity side by the sampling pitch. In this manner, the lens driving direction is determined by comparing a (n−1) and a (n) for each sampling pitch, and the operation of driving the focusing lens 11 in that direction is repeated. In the example in the figure, the focusing lens 11 is initially driven to the infinity side, but when the peak position is passed, the magnitudes of a (n−1) and a (n) are reversed, so that the peak position is exceeded. Turns out. Therefore, the focusing lens 11 is driven in the reverse direction and finally brought into the focusing range.

  As described above, in the conventional contrast AF, the peak position is known only after the focusing lens 11 passes the peak position. Therefore, it is necessary to return the focusing lens 11 by the excessive amount. This is because the contrast AF is performed based only on the output of the image sensor (corresponding to the first photoelectric conversion element 22) for obtaining image data.

  In the present embodiment, contrast AF is performed using the output of the second photoelectric conversion element 25 in addition to the first photoelectric conversion element 22. In the following description, the focus evaluation values calculated based on the outputs of the first and second photoelectric conversion elements 25 are referred to as first and second evaluation values, respectively.

  Here, the first photoelectric conversion element 22 is an image pickup element for obtaining image data to be stored, whereas the second photoelectric conversion element 25 is dedicated to AF. Therefore, the signal resolution of the second photoelectric conversion element 25 may be lower than that of the first photoelectric conversion element 22. Normally, resolution conversion (pixel thinning, etc.) is performed on the high-resolution image signal obtained by the image sensor (first photoelectric conversion element), and the focus evaluation value is calculated from the converted low-resolution image signal. By doing so, the calculation speed is increased. Since the second photoelectric conversion element 25 may originally output an image signal with a low resolution (same as the resolution after the above-described resolution conversion), the cost can be reduced by that amount, and an extra resolution conversion process needs to be performed. Therefore, the calculation speed and power saving can be improved.

FIG. 3 shows an example of AF control in the present embodiment.
When the direction in which the focusing lens 11 is to be driven is determined, the CPU 24 calculates the position of the second photoelectric conversion element 25 in accordance with the drive direction, and the photoelectric conversion element driving means 26 adds the second photoelectric conversion to that position. The element 25 is moved.

  Here, the first photoelectric conversion element 22 is fixed, and its light receiving surface is always located on the normal focal plane. On the other hand, the second photoelectric conversion element 25 can move as described above, and its light receiving surface can move back and forth with respect to a plane equivalent to the focal plane. Due to the movement of the second photoelectric conversion element 25, an optical path length difference is generated in the incident optical path of both the photoelectric conversion elements 22 and 25, and a difference is generated between the first and second evaluation values. In the example of FIG. 2, the first evaluation value when the focusing lens 11 is at the P (n) position is a (n), whereas the second evaluation value at the same position is b (n). Become. That is, the position of the second photoelectric conversion element 25 is determined so that b (n) is a value three times ahead of a (n).

Thereafter, AF search is started. The CPU 24 calculates the first and second evaluation values a (n) and b (n) at each sampling point P (n), and obtains the gradient between them. The slope is
(B (n) -a (n)) / given optical path length difference between the first and second photoelectric conversions. In the example of FIG. 2, since b (n)> a (n) at first, the gradient is a positive value.

  The gradient decreases as the focusing lens 11 approaches the peak position, and eventually the gradient becomes 0 with b (n) = a (n), or is negative with b (n) <a (n). Invert to value. Based on the change in the gradient, the peak position can be estimated before the focusing lens 11 exceeds the peak position.

FIG. 4 shows the change in the absolute value of the gradient depending on the lens position. The gradient decreases to the peak position, but increases after the peak position, and the gradient is equal immediately before and after the peak position. Therefore, based on the first and second evaluation values before and after the sign of the gradient is reversed, a position where the gradient change amount is 0 is obtained, and a peak position is obtained. The change in slope is
(Gradient (n) -gradient (n-1)) / sampling pitch. In the example of FIG. 2, since the gradient happens to be 0 at P (7), the peak position can be estimated to be the center of P (8) and P (9).

  FIG. 5 shows an example in which the lens driving direction is opposite to that described above. In this case, the second photoelectric conversion element 25 is moved to a position away from the normal focal plane in the direction opposite to the above.

  As another method, the in-focus position may be estimated based on a change in the second evaluation value b (n). For example, when the first evaluation value a (n) increases but the second evaluation value b (n) starts to decrease, the in-focus position is estimated from the second evaluation value, and The focusing lens 11 may be driven directly.

  In any of the above methods, since the in-focus position is determined before the in-focus lens 11 exceeds the peak position, there is no need to perform the operation of returning the lens once over the peak position as in the prior art, and the AF speed is increased. Can be planned.

  FIG. 6 shows another embodiment. An optical element 51 is disposed in front of the optical path of the second photoelectric conversion element 25, and light transmitted through the mirror 21 passes through the element 51 and enters the second photoelectric conversion element 25. As the optical element 51, an element having a refractive index or a lens thickness that is variable by a voltage such as a fluid lens is used, and the refractive index or the lens thickness is changed by the optical path length changing unit 52, whereby Change the incident optical path length. As a result, the optical path length difference occurs, and the same effect as the previous embodiment can be obtained. Moreover, according to this embodiment, it is not necessary to mechanically drive the second photoelectric conversion element 25.

  In the above, the incident optical path length difference between the first and second photoelectric conversion elements is constant, but this may be changed during the AF search. For example, initially the sampling pitch and the optical path length difference are increased to perform a high-speed search, and if it is found that the peak position is approached due to the change in the gradient, the sampling pitch is decreased and the optical path length difference is decreased. A highly accurate search may be performed (by bringing the second photoelectric conversion element 25 closer to the focal plane). According to this, it is possible to achieve both high speed AF and high accuracy. Alternatively, the accuracy can be improved even if the sampling pitch is constant and the optical path length difference is reduced.

  The optical path length difference may be determined by the focal depth of the lens. Since the final lens position only needs to be within the range of the focal depth, the detection accuracy of the peak position may be relatively low when the focal depth is deep, and the search speed is increased by increasing the optical path length difference. . On the other hand, the smaller the depth of focus, the smaller the optical path length difference and the higher the peak position detection accuracy.

  FIG. 7 shows an example of control in this case. When the lens driving direction is determined, lens data (focal length data) is read from the lens barrel 100. The depth is obtained from the read focal length and the aperture value, the movement amount of the second photoelectric conversion element 25 is obtained based on the depth, and the second photoelectric conversion element 25 is moved (or the optical element 51 of the optical element 51 is moved based on the depth). Change the refractive index or lens thickness). The subsequent processing is the same as in FIG.

  Both the first and second photoelectric conversion elements may be dedicated to AF, and an image sensor for obtaining image data may be provided separately. Further, three or more photoelectric conversion elements may be used, and AF control may be performed by changing the difference between the incident optical path lengths.

1 is a schematic configuration diagram of a camera according to an embodiment of the present invention. The figure explaining contrast AF control in an embodiment. 10 is a flowchart showing an example of a procedure for contrast AF. The figure which shows the change of the gradient of the focus evaluation value in a lens position. It is the same figure as FIG. 2, and shows the case where a lens drive direction is reverse. The schematic block diagram of the camera in other embodiment. The flowchart which shows the other example of contrast AF.

Explanation of symbols

DESCRIPTION OF SYMBOLS 10 Image | photographing optical system 11 Focusing lens 12 Lens drive means 22 1st photoelectric conversion element 24 CPU
25 Second photoelectric conversion element 26 Photoelectric conversion element driving means 51 Optical element 52 Optical path length changing means a (n), b (n) First and second evaluation values P (n) Sampling points

Claims (11)

A photoelectric conversion unit that receives and photoelectrically converts a subject light beam, and a calculation unit that calculates a focus evaluation value representing a focus adjustment state based on the photoelectric conversion output; In the autofocus device that calculates the focus evaluation value at each sampling position while driving, and brings the focusing lens to the focusing position based on the calculation result,
The photoelectric conversion means includes a first photoelectric conversion element having a first light receiving surface provided on a focal plane, and a second light receiving surface provided to be movable in an optical axis direction with respect to a plane equivalent to the focal plane. A second photoelectric conversion element having
A control unit that controls the position of the second light receiving surface to a predetermined position having an optical path length difference from the first light receiving surface according to the driving direction of the focusing lens;
The second photoelectric conversion element detects the peak of the focus evaluation value before the first photoelectric conversion element when the focusing lens is driven in the driving direction by the sampling pitch. Autofocus device to do. The first photoelectric conversion element is a photoelectric conversion element used for obtaining image data to be stored,
The autofocus device according to claim 1, wherein the second photoelectric conversion element is a photoelectric conversion element that is not used for obtaining image data to be stored . 3. The autofocus device according to claim 1 , wherein the difference in the incident optical path length is equal to or longer than the sampling pitch . The incident optical path length is set so that the second focus evaluation value indicates a value earlier than the first focus evaluation value in the lens driving direction. The autofocus device according to any one of the above. Said first, obtains a difference between the second focus evaluation value, the autofocus according to any one of claims 1 to 4, characterized in that the drive control of the focusing lens based on the change of the difference apparatus. The optical path length of the first photoelectric conversion element is fixed, and the optical path length of the second photoelectric conversion element is made variable, thereby generating the optical path length difference. The autofocus device according to item 1 . Resolution conversion means for converting the output of the first photoelectric conversion element and obtaining the focus evaluation value calculation signal, and the second photoelectric conversion element includes the focus evaluation value calculation image signal and 3. The autofocus device according to claim 2 , wherein the autofocus device is configured to output a signal having an equivalent resolution. Autofocus device according to claim 1, wherein determining based on said optical path length difference in the depth of focus of the imaging optical system. If the condition indicating that the focusing lens approaches an in-focus position is established, the autofocus apparatus according to any one of claims 1-8, characterized in that to reduce the optical path length difference. The means for generating the optical path length difference included in the control unit includes means for changing the position of any one of the photoelectric conversion elements, and an optical path length changing element provided in an incident optical path of any one of the photoelectric conversion elements. The autofocus device according to any one of claims 1 to 9, wherein the autofocus device is one of a means for changing a refractive index and a means for changing a thickness of the optical path length changing element. A camera comprising the autofocus device according to claim 1 .

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